The present invention relates to improvements in a wire electric discharge machining method and a wire electric discharge machine in which an electric discharge is induced in a gap formed between a wire electrode and a workpiece so that the workpiece can be machined.
Electric discharge machining has acquired a steadfast position as a technique for machining metallic dies and others. Therefore, electric discharge machining has been widely used for machining metallic dies in the automobile industry, electric appliance industry and semiconductor industry.
FIGS. 4A to 4E are schematic illustrations for explaining a mechanism of electric discharge machining. In the drawing, reference numeral 1 is an electrode, reference numeral 2 is a workpiece, reference numeral 3 is an arc column, reference numeral 4 is a machining solution and reference numeral 5 represents chips produced in the process of electric discharge machining. While the following processes (a) to (e), which correspond to FIGS. 4A to 4E, are being repeatedly conducted, removal machining is conducted on the workpiece 2 by electric discharges. Each process proceeds as follows.
(a) Formation of the arc column 3 by the generation of electric discharges
(b) Local melt of the workpiece and vaporization of the machining solution 4 by the thermal energy of electric discharges
(c) Generation of vaporizing explosive power by the machining solution 4
(d) Dispersion of the melted portion (chips 5)
(e) Cooling, coagulation and restoration of insulation between the electrodes executed by the machining solution
The present invention relates to wire electric discharge machining used for gouging, cutting and so forth. Concerning the technique of wire electric discharge machining, there is a strong demand for higher accuracy. For example, when metallic dies used in the field of manufacturing semiconductors, the dimensional accuracy of which is high, are machined, it is necessary to conduct machining with high accuracy of 1 to 2xcexcm.
FIGS. 5A to 5C are schematic illustrations showing an example of the wire electric discharge machining process. In the drawing, reference numeral 1a is a wire electrode, reference numeral 2 is a workpiece, reference numeral 4a is water which is a machining solution, and reference numeral 6 is an initial hole. FIG. 5A shows a first cut process which is a rough machining process, FIG. 5B shows a second cut process which is an intermediate finishing process to be conducted after the rough machining process, and FIG. 5C shows a third cut process which is a final finishing process.
An example of the first cut process shown in FIG. 5A shows a gouging process in which the wire electrode 1a is threaded into the initial hole 6 and the workpiece 2 is gouged by electric discharges. In the case of the first cut process described above, since the surface roughness and the accuracy are finished in the later process, it is unnecessary to machine the workpiece with smooth surface roughness high accuracy, and it is important to increase a rate of machining so as to enhance the productivity. In order to enhance the rate of wire electric discharge machining, water 4a is strongly jetted out between the electrodes so that chips can be effectively ejected from between the electrodes. In order to spray water 4a between the electrodes uniformly and prevent the breaking of the wire electrode 1a, a method is adopted in which water 4a is stored up in a machining tank not shown and the workpiece 2 is dipped in the water 4a. 
In the conventional wire electric discharge machining method described above, the second cut process (shown in FIG. 5B), which is executed after the first cut process (shown in FIG. 5A), and the third cut process (shown in FIG. 5C), are executed in the water 4a which is a machining solution.
When a voltage is impressed between the wire electrode 1a and the workpiece 2, an electrostatic force is generated between the positive and the negative polarity. By this electrostatic force, the wire electrode 1a, the rigidity of which is low, is drawn onto the workpiece 2 side, which could be a cause of vibration of the wire electrode 1a. Due to the vibration, it becomes difficult to conduct electric discharge machining with high accuracy.
When vaporizing explosive power is generated in the machining solution by electric discharge energy (for example, as shown in FIG. 4C), the wire electrode 1a is given a strong force by the vaporizing explosive power created in the machining solution in a direction opposite to the workpiece 2, and vibration is generated. Due to the vibration, irregularities are caused on the profile of the workpiece 2, which impairs the dimensional accuracy.
In the industry of semiconductors in which wire electric discharge machining is utilized, the following uses are increased. For example, in the case of machining metallic dies of IC lead frames, very high accuracy and very smooth surface roughness are required when a workpiece is machined, the dimensional accuracy of which is not more than 1 xcexcm, and the surface roughness of which is not more than 1 xcexcm Rmax, and further it becomes necessary to enhance the productivity. Especially, in the case where high accuracy and smooth surface roughness are required as described above, remarkable problems are caused by the aforementioned vibration of the wire electrode.
As a means for solving the above problems caused in wire electric discharge machining conducted in the machining solution, there is disclosed a technique of wire electric discharge machining conducted in the atmosphere while a machining solution is not being interposed between the electrodes. This technique is disclosed by the title of xe2x80x9cEnhancement of Accuracy of Second Cut by Electric Discharge Machining Conducted in Atmospherexe2x80x9d written by Adachi et al. of Tokyo University of Agriculture and Technology on page 154 of the seventh issue of the fourteenth volume of xe2x80x9cDie Technologyxe2x80x9d published by Nikkan Kogyo Shinbunsha in 1999. According to this technique, it is disclosed that the straightness of a cut section of a workpiece can be enhanced by wire electric discharge machining conducted in the atmosphere. From the viewpoint of enhancing the accuracy, this technique has a great significance. However, it is difficult to put this technique into practical use because a short circuit tends to occur between the wire electrode and the workpiece compared with the electric discharge machining conducted in a machining solution and the machining can not be stabilized and the rate of machining can not be increased.
It is an object of the present invention to provide a wire electric discharge machining method and a wire electric discharge machine capable of realizing the high accuracy and quality of wire electric discharge machining and also capable of realizing the enhancement of productivity of wire electric discharge machining.
The present invention provides a wire electric discharge machining method in which electric discharge is generated between the electrodes of a wire electrode and a workpiece so as to machine the workpiece by the electric discharge, comprising the steps of: a first step of conducting rough machining in a machining solution; and a second step of conducting finish machining in gas, wherein a relative traveling speed of the wire electrode with the workpiece in the second step is set at a constant speed not lower than a predetermined speed at which a short circuit between the electrodes can not continue for a predetermined period of time and more which has been previously set according to a required specification.
The present invention also provides a wire electric discharge machining method in which electric discharge is generated between the electrodes of a wire electrode and a workpiece so as to machine the workpiece by the electric discharge, comprising the steps of: a first step of conducting rough machining in a machining solution; and a second step of conducting finish machining in gas, wherein a relative traveling speed of the wire electrode with the workpiece in the second step is set at a variable speed not lower than a predetermined speed at which a short circuit between the electrodes can not continue for a predetermined period of time and more which has been previously set according to a required specification.
The present invention also provides a wire electric discharge machining method, in which the variable relative traveling speed is controlled so that it can be increased in the case where the number of normal electric discharge pulses per unit time is large, and the variable relative traveling speed is controlled so that it can be decreased in the case where the number of normal electric discharge pulses per unit time is small.
The present invention also provides a wire electric discharge machining method, in which the predetermined traveling speed at which the short circuit can not continue for a predetermined period of time and more which has been previously set according to the required specification is compared with the predetermined traveling speed at which a frequency of concentrated generation of electric discharge is lower than a frequency which has been previously set according to the required specification, and the relative traveling speed is controlled so that it can be a speed not lower than the faster speed of the two traveling speeds.
The present invention provides a wire electric discharge machine in which electric discharge energy is supplied between a wire electrode and a workpiece by a machining electric power supply means so as to machine the workpiece in gas when the wire electrode and the workpiece are relatively traveled with each other by a positioning means, comprising a control means for controlling the positioning means so that a relative traveling speed of the wire electrode with the workpiece can be set at a constant speed not lower than a predetermined speed at which a short circuit between the electrodes can not continue for a predetermined period of time or more which has been previously set according to a required specification.
The present invention also provides a wire electric discharge machine in which electric discharge energy is supplied between a wire electrode and a workpiece by a machining electric power supply means so as to machine the workpiece in gas when the wire electrode and the workpiece are relatively traveled with each other by a positioning means, comprising a control means for controlling the positioning means so that a relative traveling speed of the wire electrode with the workpiece can be set at a variable speed not lower than a predetermined speed at which a short circuit between the electrodes can not continue for a predetermined period of time and more which has been previously set according to a required specification.
The present invention also provides a wire electric discharge machine, further comprising a control means for controlling the variable relative traveling speed so that it can be increased in the case where the number of normal electric discharge pulses per unit time is large and also for controlling the variable relative traveling speed so that it can be decreased in the case where the number of normal electric discharge pulses per unit time is small.
The present invention also provides a wire electric discharge machine, further comprising a control means for controlling the positioning means in such a manner that the relative traveling speed is controlled so that it can be a speed not lower than the faster speed of the two traveling speeds when the predetermined traveling speed at which the short circuit can not continue for a predetermined period of time and more which has been previously set according to the required specification is compared with the predetermined traveling speed at which a frequency of concentrated generation of electric discharge is lower than a frequency which has been previously set according to the required specification.
Since the wire electric discharge machining method and the electric discharge machine of the present invention are composed as described above, it is possible to enhance the stability of wire electric discharge machining conducted in gas and further it is possible to increase the rate of machining. Therefore, the accuracy, quality and productivity of wire electric discharge machining can be enhanced.